The present invention relates generally to solid sorbents for removing hydrogen sulfide values from hot process gas streams containing such values, and more particularly to a multiple metal-oxide sorbent supported on a silicon oxide substrate for removing hydrogen sulfide from a stream of hot gaseous products resulting from the gasification of coal. The United States Government has rights in this invention pursuant to the employer-employee relationship of the U.S. Department of Energy and the inventors.
The gasification o sulfur-bearing coal produces various sulfur-containing compounds in gaseous form with hydrogen sulfide being the major compound produced. The discharge of such hydrogen sulfide into the atmosphere causes grave environmental problems due to the conversion of the hydrogen sulfide to sulfur oxides which leads to the formation of acid rain as well as causing other environmental problems.
Clean-up procedures for removing essentially all of the hydrogen sulfide from process gas streams has been satisfactorily accomplished by using several different techniques. These techniques range from costly and thermally inefficient liquid scrubbers to the use of solid metal sorbents such as iron oxide, copper oxide, and zinc-based sorbents inducing zinc oxides, zinc ferrite, and zinc titanate. The utilization of these solid sorbents, especially the zinc-based sorbents, were found to be particularly efficient for removing hydrogen sulfide from relatively hot process gas streams such as provided by the gasification of coal. By removing essentially all the hydrogen sulfide values from the hot gas stream, the residual heat remaining in the gases could be used in a thermal expansion device such a s a gas turbine. This removal of hydrogen sulfide from such hot gas streams for use in hot gas utilization devices is not only important from an environmental standpoint but is required for inhibiting the deleterious degradation effects the hydrogen sulfide has upon metal surfaces contacted thereby.
The combination of a coal gasifier with a hot gas turbine has been found to be a highly efficient and economical system for deriving energy from coal, particularly in the form of electrical energy as provided by a gas turbine-electrical generator hook up. The efficiency of such combined-cycle systems increases as the gas temperature increases within a range from about 1200.degree. to about 1600.degree. F., preferably about 1400.degree. to about 1600.degree. F. or even higher. However, when attempting to remove hydrogen sulfide from coal-gasifier produced gases at temperatures greater than about 1200.degree. F. and especially greater than about 1400.degree. F., by using presently known metal and metal oxide sorbents, the results have not been particularly satisfactory. These sorbents, except for possibly copper oxide on a silicon oxide substrate, undergo considerable structural degradation and also exhibit a substantial loss in efficiency. For example, while zinc ferrite is capable of reducing the hydrogen sulfide content in gases produced by the gasification of coal to a level less than 10 ppmv at temperatures below about 1200.degree. F., this sorbent undergoes considerable decrepitation, loss of pore volume, and reactivity at temperatures greater than about 1200.degree. F. Zinc titanate is also a satisfactory sorbent at relatively high temperatures since the titanium oxide phase helps to stabilize the zinc oxide phase to reduce the volatilization thereof at temperatures greater than about 1290.degree. F. However, even with this stabilization feature zinc titanate was found to have a relatively low capacity for hydrogen sulfide since the latter does not react with the titanium oxide. Thus, the capacity of zinc titanate for hydrogen sulfide is lower than that provided by zinc ferrite due to the dilution effect provided by the titanium oxide even though zinc titanate can withstand temperatures up to about 1350.degree. F. while maintaining good structural integrity. Thus, the use of zinc titanate at such temperatures or higher is relatively limited since its sulfur capacity is relatively low, i.e., in the range of about 17-24 gram sulfur per 100 grams of sorbent compared to the 40 grams of sulfur per 100 grams of zinc ferrite.
Other previously known sorbents include copper manganate sorbents in bulk form which perform relatively well up to about 1400.degree. F. where the sulfur capacity of this sorbent decreases rapidly, even in the absence of steam. Copper oxide on a support such as a zeolite was previously found to be a good sulfur sorbent at a relatively high temperature of about 1100.degree. F. As described by the investigators in the publication "Removal of H.sub.2 S From Hot Gas in The Presence of CU-Containing Sorbents", Fuel, Volume 68, No. 1. (1989), Takashi Kyotani et al, copper oxide was not considered to be an efficient sorbent because a surface sulphide layer formed on the outer surface of the copper oxide pellets and limited the utilization of the active copper. However, it was determined that almost complete copper utilization was achieved by combining copper oxide with supports such as silicon oxide and a zeolite.
Further testing of copper oxide sorbents supported on silicon oxide substrates has since been conducted at temperatures as high as 1600.degree. F. As will be discussed below, these tests showed that while the copper oxide sorbent supported on silicon oxide did not undergo undesirable degradation at high temperatures, the efficiency and capacity of the sorbent was not believed to be adequate for for hot gas clean-up in combined-cycle systems. SUMMARY OF THE INVENTION
Accordingly, it is a object of the present invention to substantially increase the efficiency and capacity of a copper oxide sorbent supported on a silicon oxide substrate by admixing with the copper oxide one or more additional metal oxides.
Another object of the present invention is to provide a multiple metal oxide sorbent on a silicon oxide support for efficiently removing hydrogen sulfide values from process gas streams, especially as provided by the gasification of coal, at elevated temperatures where previously known sorbents such as generally described above were rendered substantially ineffective or inefficient for extended use at such temperatures.
Another object of the present invention is to support a mixed metal oxide sorbent on a substantially silicon oxide zeolite and provide a sorbent arrangement which undergoes minimal structural degradation or loss of efficiency and capacity at temperatures in the range of about 1200.degree. to about 1600.degree. F., especially at temperatures nearer the high end of this temperature range.
Generally, the sorbent of the present invention for removing hydrogen sulfide values from a process gas stream at elevated temperatures comprises a mixture of copper oxide, an effective amount of manganese oxide to promote the reaction of hydrogen sulfide with the copper oxide, 0-40 weight percent molybdenum oxide, and a substrate of substantially silicon oxide supporting the mixture.
In a modification of the sorbent of the present invention, the sorbent comprises a mixture of copper oxide, molybdenum oxide in a concentration effective to promote the reaction of hydrogen sulfide with copper oxide, manganese oxide in a concentration effective to inhibit volatilization of the molybdenum oxide, and a substrate of substantially silicon oxide supporting the mixture.
Hydrogen sulfide values are removed from a stream of gaseous products resulting from the gasification of coal at a temperature greater than about 1200.degree. F. by contacting of the stream of gaseous products with a sorbent provided by a silicon oxide supported mixture of copper oxide, an amount of molybdenum oxide effective to promote the reaction of hydrogen sulfide with the copper oxide at the selected temperature, and an amount of manganese oxide effective to further promote the reaction of hydrogen sulfide with copper oxide and/or to inhibit volatilization of the molybdenum oxide at the selected temperature.
Other and further objects of the present invention will become obvious upon an understanding of the illustrative embodiment and method about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.